Step 1. Writing/Compiling/Running Java code

1. Using your favorite text editor (as long as it's emacs or vi) put the following Java code into a file named Lab1a.java:

   public class Lab1a 
   {
     public static void main(String[] args) 
     {
           
     }
   }

   Note: The name of the file must be exactly Lab1a.java. Yes, upper-case vs. lower-case matters!
2. Add the following to the main method (aka function):

   int[] x = new int[10];
   int k = 0;
   while(K < 10) 
   {
      x[k] = k*k;
      k++;
   }
   for (int j = 0; j < 10; j++) 
   {
      System.out.println(j + " squared is " + x[j]);
   }

   What do you think it will do when run?
3. Now it is time to compile this program. You do this by typing the command

   javac Lab1a.java

   (javac = "java compiler"). If you typed it in exactly as above, you should have gotten an error. Take a look at it, and notice how helpful javac is being. Fix the error.
4. Recompile the program (so you got no errors). If you type

   ls

   you'll notice a new file, Lab1a.class, which is the bytecode to be run by the java interpreter. Run the interpreter on your program with the command

   java Lab1a

5. Modify the program by replacing the while loop with a for loop. Compile it and run it again.

Step 2. Your first Program

1. Make a new file called Lab1b.java. Write the lines you need to write to make it a Java program.
2. Add the line

   import java.util.*;

   at the very top of the file. This is like #include <iostream> in C++. It allows us to use the Scanner class, which is what we'll use to read input.
3. Add the following code to main. Run to see what it does:

   Scanner in = new Scanner(System.in);
   System.out.print("Please input an integer ");
   int n = in.nextInt();
   System.out.print("Please input a second integer ");
   int k = in.nextInt();
   System.out.println("The two ints were " + n + " and " + k);

4. What happens if you type your name instead of an integer? Try it! What you should see is that the JVM prints out the call stack, i.e. the stack of function call records as it appeared when the error occurred. One nice feature of an interpreted language is that when a run-time error occurs, the interpreter can tell you all sorts of things about the state of the program when the error occurred. This is really nice. For programs running on the CPU, the only information you get when a run-time error occurs is a message like "seg-fault".
5. Scanner is a class that is part of Java's utility package (java.util). A package is "roughly" analogous to a C++ library. Scanner is used to facilitate reading information from an input stream. In this case, we are reading from System.in which is that standard input stream. In order to use the Scanner class, you have to import the java.util package. That's the purpose of the 'import' line of the file.
6. Did you know that if you want to play a simple lottery where there are n possible numbers (the number of balls), and you have to pick k of them, then your odds of winning are 1 in (n * (n-1) * (n-2) * ... * (n-k+1))/k!, where ! means "factorial". If not, don't worry. Your probs and stats course will teach you these kinds of things. Just accept this formula for purposes of the lab.
7. Anyway, modify your program so that it reads in the n and k, and computes the odds as described above. You should be able to do this with code that (not counting I/O) looks just like C++.
   Note: to cast an int x to a double in Java you do this (double)x ... just sayin'.

   ~/$ java Lab1b Please input an integer 8 Please input a second integer 5 The two ints were 8 and 5 numerator = 6720 denomintor = 120 odds = 1 in 56 = 0.017857142857142856

   ~/$ java Lab1b Please input an integer 9 Please input a second integer 1 The two ints were 9 and 1 numerator = 9 denomintor = 1 odds = 1 in 9 = 0.1111111111111111

   ~/$ java Lab1b Please input an integer 13 Please input a second integer 7 The two ints were 13 and 7 numerator = 8648640 denomintor = 5040 odds = 1 in 1716 = 5.827505827505828E-4

8. You may wonder what else the Scanner class can do. The Scanner class, and much of the rest of what is built into Java, is described in your book. Documentation for the complete "Java API", i.e. descriptions of all packages/classes that are part of the language standard, is available online: Java 11 API Documentation. To find Scanner, use the search box on the upper right. (Or google jdk11 scanner.) You'll notice a long list of functions included in the Scanner class. Which one do you think will read in a whole line? What type does it return? Change your code so that before the integers are entered, the user enters name (spaces allowed!), which is printed back in the final println() command.

   ~/$ java Lab1b Enter your name: Alfred E. Newman Please input an integer 11 Please input a second integer 4 The two ints were 11 and 4 numerator = 7920 denomintor = 24 odds = 1 in 330 = 0.0030303030303030303 Goodbye Alfred E. Newman.

   ~/$ java Lab1b Enter your name: Roger Federer Please input an integer 14 Please input a second integer 5 The two ints were 14 and 5 numerator = 240240 denomintor = 120 odds = 1 in 2002 = 4.995004995004995E-4 Goodbye Roger Federer.

9. Add a function which takes an int and returns the factorial of that integer. Place that function immediately before the main function, i.e. still inside the class definition. Functions (for now) should start with the magic incantation public static, otherwise they look just like C++ functions. Note that, unlike C++, there are no function prototypes separate from function definitions. Test your function before continuing on.
10. Add another function which, given n and k from the above program, returns the product n*(n-1)*...*(n-k+1).
11. Modify your main function to use these two new functions to calculate and output the odds from the above section.

Submission instructions

submit -c=IC211 -p=lab01 Lab1b.java Lab1c.java